Coated metal article and method of making same



Patented May 11, 1943 COATED METAL ARTICLE AND METHOD OF MAKING SAltIEJohn S. Thompson, Detroit, Micln, assignor to Parker Rust Proof Company,Wayne County,

Mich.

No Drawing. Original application April25, 1941, Serial No. 390,337.Divided and this application September 14, 1942, Serial No. 458,333

13 Claims.

This invention relates to an article having a surface of zinc, or analloy of zinc, and having on said surface a coating of paint, enamel,lacquer, or similar siccative material bonded to the surface by a thin,continuous, chemically formed, visible, adherent coating containingoxide of the metal of the surface, and to a method of producing such anarticle.

This application is a division of my application of the same title filedApril 25, 1941, Serial No. 390,337, which in turn is a continuation inpart of my former application for Coated article and method of makingthe same, Serial No. 260,741, filed March 9, 1939, which in turn was acontinuation in part of prior applications, including application SerialNo. 45,792, filed October 19, 1935, and this application is also acontinuation in part of my former applications Serial No. 97,298, filedAug. 21, 1936; of applicatlon No. 42,667, filed Sept. 28, 1935, in whichis retained claims specific to the use of compounds of hexavalentchromium in the original coating solution: of application No. 42,669,filed Sept. 28, 1935; of application No. 42,670, filed Sept. 28, 1935,issued Mar. 17, 1942, as U. 8. Patent No. 2,276,353, and in which isretained claims specific to the use of the SlFs radical in the coatingsolution; and of application No. 158,706, filed Aug. 12, 1937, which inturn was a continuation in part of said application 97,298; and isgeneric in some respects to my joint application with Edward S. Hanson,Serial No. 85,335 filed June 15, 1936, which issued July 29, 1941, as U.S. Patent No. 2,250,508, which contains specific claims to coating zincin solution containing certain organic acids.

The bonding coating to which the invention relates is a :thin coating asdistinguishing from a relatively thick phosphate or similar coating ofsalts of the metal being coated, although some of such salts may bepresent in the thin coating. It is a visible coating as distinguishedfrom a coating of oxide, or the like, which may be sufllcient topassivate the metal to some extent, .but leaves the surface with itsoriginal metallic appearance.- While the coating may be somewhatvariable in thickness, it is preferably substantially continuous so thatit bonds the 'siccative coat to all portions of the metal surface. Itmay contain compounds of a metal other than the metal of the surface,but:if so, these compounds are given their final form by chemicalreaction at the surface of the-work in connection with the oxidation ofsome of the metal of the surface so that the coating is integrally boundto the surface of the metal.

The bonding coat is produced by a dilute aqueous solution of an etchingacid for the metal being treated, and an oxidizing agent for the metalbeing treated. The solution may also contain a compound of a metal whichis capable of forming an insoluble oxide which may constitute a part ofthe coating. 1

A wide variety of etching acids may be employed, but the successfulproduction of the coating depends upon employing the acids in such astrength that there will be a promntreaction with the metal surfacebeing treated, but that this etching action shall not be too violent.Various oxidizing agents may be employed which are compatible with theetching-acid in the solution, the amount of the oxidizing agent beingsuch that the metal of the surface will be oxidized promptly as it. isetched. By the phrase an oxidizing agent for the metal being treated ismeant an agent that acts as an oxidizing agent in facilitating theformation in the coating of a substantially insoluble oxide of the metalbeing treated. The etching and oxidizing action should be sumcientlyactive so that a satisfactory coating can be produced in not over fiveminutes. In actual operation, usually one minute is sufficient time.This rapid reaction produces a different coating than that which couldbe produced by subjecting the surface to a prolonged treatment, such asa half hour or more, in an acidulated solution containing an oxidizingagent.

The action of the solution can be expedited frequently, and sometimesthe quality of the coating can be improved, by the inclusion in thesolution of a compound that is soluble in the solution but contains ametal which may form an oxide substantially insoluble in the solution.

This is true of substantially all of the metals in the electromotiveseries below magnesium and' above hydrogen, and includes'some belowhydrogen, such as arsenic for example.

The oxidizing agent and acid, when properly proportioned and employedupon a ferrous surface, result in producing a gray oxide which maydarken in colorrto approach a black. 'This is a different character ofoxide from that produced by ordinary rusting, for example, and appearsmore like the oxide produced on iron at temperatures above those usedfor producing. a blue oxide on iron. I

A chemically produced coating of the character described improves thesurface of the metal as a base for a slccative coat; but-the adherenceof the siccative coating and the resistance to corrosion of thecompleted article is generally enhanced to a marked degree by anintermediate treatment of the chemical coating with a dilute solution ofa compound of hexavalent chromium before the siccative coating isapplied.

Having indicated the general nature of the invention, specific examplesof the manner of carrying out the invention will be given in detail.

Example 1.-To one gallon of water is added 20 cc. of 75%ortho-phosphoric acid and 20 grams of sodium chlorate.- When used at 100F. and sprayed onto the metal to be coated through the use of amechanical spraying machine this solution is found to coat iron or steelin less than one minute. 0.002% or more silver in the form of silversulfate; 0.002% copper, or more, added in the form of copper sulfate;0.002 manganese, or more, added in the form of manganese sulfate ormanganese dihydrogen phosphate, may be added to the solution withbeneficial results.

Example 2.-Increasing the phosphoric acid in Example 1 to 30 cc. givessomewhat better results under the conditions there described. When usedat 170 F. for one minute, 20 cc. of phosporic acid to 5 grams ofpotassium or sodium chlorategave very good results.

Example 3.-Using a solution of exactly the same composition as is shownin Example 1, a coating is produced on a zinc surface in one minute.

Example 4.-A solution is made up composed of one gallon of water, 20 cc.of 75% ortho-phosphoric acid and 8 cc. 30% hydrogen peroxide. Anadherent coating is obtained on iron or steel when treated with thissolution for one minute at 100 F. The addition of 1 gram of sodiumarsenate to this solution results in a blue-black coating in one minute.

Example 5.A solution composed of one galion of water, 20 cc. of 95%sulfuric acid and 20 grams of sodium chlorate produces an adherent oxidecoating on iron or steel in one minute when used at 100 F. and appliedby means of a mechanical spraying machine. when 5 grams aluminum sulfatewere added to this solution a coating resulted on iron which proved tobe even more corrosion resistant. 1

Example 6.--A solution composed of one gal- 5 ion of water, 20 cc. of95% sulfuric acid, 20, grams of potassium chlorate and 2.5 grams ofchromic sulfate produces an oxide coating on iron or steel in one minutewhen applied to the metal surface by means of a mechanical sprayingmachine at 100 F. Other chromic saltsmay be substituted for the, chromicsulfate in the same proportion. In place of the chromic salt may be used18 grams of zinc dihydrogen phosphate. 0.7 gram of M003, 0.5 gram ofbasic lead carbonate or 1.6 grams of sodium arsenate.

Example 7.A solution composed of 1,000 cc.-

water, 10 cc. 75% phosphoric acid and 2 grams sodium bromate produces auniform coating on iron or steel in 1 minute at 100? F. when the metalis immersed in the solution.

Example 8.An adherent coating is formed upon iron and steel whensubjected to a spray of a solution containing cc. of 50% hydrofluoricacid and 20 grams of sodium chlorate per gallon of water at 100 F.

Example 9.A solution containing 20 cc. of 75% ortho-phosphoric acid and10 grams of quinone per gallon of water satisfactorilycoats iron orsteel surfaces in one minute when applied by means of a mechanical spraymachine.

Example 10.The treatment of iron or steel surfaces in a solutioncontaining grams of 5 acetic acid, 20 cc. of 75% ortho-phosphoric acidand 20 grams of sodium chlorate per gallon of water results in anadherent oxide coating in one minute.

Example 11 .-A steelpanel was processed for five minutes in a solutionof 18 cc. H1804 and 18 grams mm in a gallon of water. A soft, dark gray,coating resulted on the surface.

Example 12.A steel object was immersed five minutes in a boilingsolution of 36 grams sodium 15 chromate, 18 cc. sulfuric acid, and 36grams nickel sulfate in a gallon of water. A soft black coating wasobtained. A more adherent coating was produced by the addition of sodiumnitrate.

In general, solutions given as suitable for treat- 20 ing iron may beemployed in treating zinc, although some changes in proportions areadvisable because of the differences in the readiness with which ironand zinc, respectively, react with the various chemicals.

A few specific examples for treating zinc will be given.

Example 13.A dark coating was produced on galvanized iron by immersionfor three minutes a at 125 F. in a solution containing 20 grams of 0NaClOa and 20 cc. sulfuric acid per gallon and 0.05% of ferric iron,introduced as sulfate.

Example 14.-A dark coating was produced on galvanized iron by immersionfor two minutes at 125 F. in a solution containing 20 grams NACIO: and20 cc. sulfuric acid per gallon and 0.04% Ni, introduced as carbonate.

Example 15. -A dark coating was produced on galvanized iron by immersionat 125 F. in a solution containing 20 grams NaClOa and'20 cc. sulfuricacid per gallon and 0.02% cobalt introduced as sulfate.

The time, temperature and strength of in gredlents required varies withthe nature of the zinc surface being treated. A galvanized surface onwhich bright spangles appear is not coated as readily as a comparativelydull surface. The above examples are given by way of illustration andnot as formulae which must be followed ac curately, since widevariations in proportions of ingredients, as well as substitutions, maybe made within the scope of the general principles disclosed herein.

While many different accelerating metals may 5 be employed in producingoxide coatings on iron or zinc, it is found that, in coating iron, thepresence of zinc in the solution aids in the prompt formation of acoating, and the presence of aluminum or trivalent chromium results inexceptionally good results under paint. In coating, zinc, iron, nickeland cobalt have proven to be exceptionally good.

Apparently the iron oxide in the least partially magnetic.

coatings is at This is difllcult to ascertain for certain when iron iscoated, because the iron itself is magnetic, but when zinc is properlycoated in a solution containing iron, the magnetic quality of thecoating is easily demonstriated.

Other etching acids, or mixtures thereof, may be substituted in theabove solutions. Slight variations in the proportions'may be advisablewith different acids, although in general quite wide variations in theproportions of acids are 75 permissible.

Other oxidizing agents, including strong agents such as persulfate toweak agents such as sulflte. may be used. Here, likewise, some variationin proportion is advisable according to the nature of the oxidizingagent employed, but quite an excess of oxidizing agent, over thatnecessary to produce the coating, may be used without harmful results.

Of course, the acid and oxidizing agent should be chosen so that theywill both remain in solution and function as an etching acid and anoxidizing agent.

When an acid is employed which is capable of forming both soluble andinsoluble salts of the metal being treated and/or of any acceleratingmetal in the solution, there is likely to be some of the insoluble saltin the coating. With inorganic acids, the insoluble salt generallycontains less acid in proportion to the metal than does thesoluble-salt. For example, phosphates of iron increase in solubility asthe proportion of acid to iron in the salt is increased. Probably thisaccounts for the observed fact that the proportion of the salt to oxidein the coating increases as the acid in the solution becomes moredilute.

The following data as to variations in treating solutions and resultantcoatings is provided to assist in proportioning the solutions so as toobtain a coating containing a material amount of the oxide of the metalbeing coated.

Tapas:

Temper- Solution per gallon 8mm Time F. Jilinuics (l) 5 cc. 75% H1PO4+mg. NaC-lO: 100 2 (2) 8 O0. 75% HsP04+20 g. NaClOa. 100 2 (3) 12 cc. 75%HaPOr-i-N a. N'aC-lOa 100 2 (4) 14 cc. 75% HsPOi-i-Z) E. NaClO; 100 2(5) 16 cc. 75% HaPO4+20 g. NaClOg. 100 2 (6) 18 cc. 75% H5P04+20 g.NaClO 100 2 (7) 18 cc. 76% H3POl+m g. NaCl i 1m 2 (8) 30 75% HaPOH-ZO g.NaClO: l!) 2 (9) 30 cc. 75% HsPOr-HO g. NaClOa 160 2 TAaLu-ContinuedCoating analysis Fe Fe PO.

. Per cent Per can! Per cent 4,50 38. 23 17. 61 15. 04 i7. 48 6. M 14.76 40. 58 4. 34 9. 93 45. 80 3. 82 9 84 48. 03 3. 50 3. 84 47. 49 1. P64. 16 ii. 75 25. 33 2. 79 48. 68 1.58 l7. l i0. 02 11.30

The trend is obviously that as the amount of phosphoric acid in solutionincreases the percentage of phosphate in the coating decreases, whilethe percentage of ferric iron increases. The chart also shows that wherethe temperature is raised the amount of phosphate inthe-coatingincreases surprisingly. The remainder of the coatings shown in the chartis oxide.

In the light of the foregoing description, iron or zinc may be providedwith a chemical coating in accordance with this invention. This coatingis not recommended as the final finish, but it is recommended that it becovered with a suitable siccative coat, such as paint, lacquer, orenamel. Also, an intermediate treatment of the chemical coating beforethe application of the siccative coating is recommended for bestresults.

In nearly every instance, added corrosion resistance and betterpaint-holding qualities are obtained by a treatment of the. metalarticle, after being coated in accordance with the foregoingdescription, in a solution of a compound of hexavalent chromium. Forthis purpose. a solution containing as little as 2 grams of C20: pergallon of water is satisfactory; For convenience, this solution will bereferred to as a rinse solution. The rinse solution may be applied tothe coated surface in any suitable manner, but it is usually best toapply it in the same manner as that used for applying the coatingsolution. It may be used at any temperature from room temperature up toboiling. The time of treatment may vary'with the strength of thesolution, the temperature at which it is applied, the character of thefirst coating, etc., but a treatment for one minute is usually ample toproduce the desired results.

Where CrOa is employed as the hexavalent chromium compound, 2 grams pergallon will produce good results, but this amount can be increasedmaterially without harmful results. From 2 grams to 20 grams CrOa pergallon is recommended. Too strong a solution may leave discoloredstreaks, and the solution reacts with the first coating anddissolvessome compounds from some of the coatings quite rapidly. Somereaction is desirable and apparently necessary to obtain the bestresults, but it is preferred to so limit the strength of the solutionand the time I and temperature of its application that the appearance ofthe first coating is not materially altered.

When the treatment with the hexavalent chromium solution is quitevigorous or prolonged, the remnant of the solution may be rinsed oilbefore the siccative coating is applied, but generally it is more.convenient and satisfactory to let the remnant of the solution clingingto the work dry thereon, and good results are obtained by drying underheat.

Usually the major part of the chromium remaining on the coating fromthis treatment is reduced in situ to the trivalent form. Usually some ofthe chromium remains in hexavalent form, but this constitutes a minoramount of the chromium, and good results are obtained where all of thechromium is reduced by the time the siccative coat is thoroughly dried.

A solution of C10: is preferred for the rinse solution, but it does nothave to be a pure solution of CrOa or of hexavalent chromium compounds.In processing, not only chromates and dichromates, but other impuritiesare usually built up in the solution, but it is not necessary to purifythe solution until it is evident that the contamination has some effectupon the coating. Any'solution containing chromic acid may be employedin the rinse provided, of course, that care be taken that the otheringredients of the rinse solution do not injuriously affect thecoat-.

portions is permissible, but it is recommended for commercial purposesto stay within the proportions of 1 cc. to 4 cc. of the acid to 2 gramsof chlorate or its equivalent.

For replenishing purposes, a similar solution may be provided. Whenphosphoric acid is employed, phosphates formed by etching the metalbeing coated are deposited as sludge and to some degree enter thecoating, so that the acid is depleted more rapidlythan'is the oxidizingagent. In coating iron with a solution of phosphoric acid and chlorate,it has been foundnecessary to replenish the solution with from 2 to 6cc. of phosphoric acid to each gram of chlorate, and therefore areplenishing solution for such a treatment should be made up withinthose proportions.

Initially, the phosphate formed by etching the surface is soluble, andsoluble phosphates may be added deliberately, as indicated in Examples 1and 6. The presence of such phosphates in the solution tends to increasethe amount of phosphates in the coating, but a sufficient amount of astrong oxidizing agent, such as hydrogen peroxide, chlorate, orpermanganate, will result in a material amount of oxide in the coatingeven with considerable increase of soluble phosphate over that indicatedin the examples.

While numerous variations in the character of solutions employed and intheir manner of application have been described above, it will beunderstood that these are by way of illustration;

and do not attempt to set forth all variations which are possible withinthe broad principles of this invention and within the scope of theappended claims.

What I claim is:

1. An article with a surface containing zinc carrying a chemically boundcoating which is predominantly oxide and contains zinc oxide, and whichalso contains hexavalent chromium.

2. An article with a surface containing zinc and having on said surfacea thin continuous adherent protective and bonding coating chemicallybound to the surface and being predominantly oxide, containing zincoxide and also containing triy-alent chromium reduced in situ, saidchemically bound coating being covered with a siccative coating.

3. An article of metal selected from the class consisting of zinc andits alloys and having formed thereon a thin continuous adherentprotective and bonding coating composed essentially of an oxide of zincand of an oxide of another metal below magnesium and above hydrogen'inthe electromotive series, said coating being formed by reaction of themetal of the article with an acidulous aqueous solution of an oxidizingagent for said metal and a salt of said other metal, said coating alsocontaining trivalent chromium reduced in situ, and having a siccativecoating covering said coating formed by reaction of the metal.

4. An article having a surface of zinc and having formed on said surfacea thin continuous adherent protective and bonding coatingcomposed-essentially of oxides of iron and zinc, said coating beingformed by reaction of the metal of the article with an acidulous aqueoussolution of an oxidizing agent for said metal and an iron salt, andhaving a siccative coating covering said coating formed by reaction ofthe metal.

5. An article having a surface of zinc and having formed on said surfacea thin continuous adherent protective and bonding coating composedessentially of oxides of nickel and zinc,

said coating being formed by reaction of the metal of the article withan acidulous aqueous solution of an oxidizingagent for said metal and anickel salt, and having a siccative coating covering said coating formedby reaction of the metal.

6. An article having a surface of zinc and having formed on said surfacea thin continuous adherent protective and bonding coating composedessentially of oxides of cobalt and zinc, said coating being formed byreaction of the metal of the article with an acidulous aqueous solutionof an oxidizing agent for said metal and a cobalt salt, and having asiccative coating covering said coating formed by reaction of the metal.

7. An article havinga surface of zinc and having formed on said surfacea thin continuous adherent protective and bonding coating.composedessentially of oxides of iron and zinc, said coating being formed byreaction of the metal of the article with an acidulous aqueous solutionof an oxidizing agent for said metal and an iron salt, the iron oxidebeing present principally in the form of the magnetic oxide, and havinga siccative coating covering said coating formed by reaction of themetal.

8. An article having a surface of zinc and having formed on said surfacea thin continuous adherent protective and bonding coating composedessentially of oxides of iron and zinc, said coating being formed byreaction of the metal of the article with an acidulous aqueous solutionof an oxidizing agent for said metal and an iron salt, said coating alsocontaining trivalent chromium reduced in situ and having a siccativecoating covering said coating formed by reaction of the metal.

9. The method of coating a surface of metal of the class consisting ofzinc and its alloys, which comprises treating said surface with anaqueous solution containing an etching acid for the metal being treatedand an oxidizing agent for the metal being treated, proportioning theacid and oxidizing agent to form an oxide of the metal of the surface asit is etched at a rate to produce a thin continuous visible adherentpredominantly oxide coating on said surface in not more than fiveminutes, continuing the treatment until said continuous coating isproduced, thereafter treating said chemical coating with a dilutesolution of a compound of hexavalent chromium, and thereafter coveringsaid continuous chemically formed coating with a siccative coating.

10. The method of coating a zinc surface which comprises treating saidsurface with an aqueous solution of an etching acid for zinc. anoxidizing agent for zinc, and a soluble compound of a metal betweenmagnesium and hydrogen in the electromotive series, proportioning theacid and oxidizing agent to form an oxide of the zinc of said surface asit is etched and to'produce a thin continuous visible adherentpredominantly oxide coating on said surface in not more than fiveminutes, said coating containing oxide of zinc and also containing metalfrom said compound in the solution, continuing the treatment until saidcontinuous coating is formed, thereafter treating the continuouschemically formed coating with a dilute solution'of a compound ofhexavalent chro-' mium, and thereafter applying a siccative coating tosaid continuous chemical coating.

11. The method of coating a zinc surface which comprises treating saidsurface with an aqueous solution of an etching acid for zinc, anoxidizing agmt for zinc, and a soluble compound of iron, proportioningthe acid and oxidizing agent to form an oxide of the zinc of saidsurface as it is etched and to produce a thin continuous visibleadherent predominantly oxide coating on said surface in not more thanfive minutes, said coating containing oxides of zinc and iron,continuing the treatment until said continuous coating is formed,thereafter treating the continuous chemically formed coating with adilute solution of a compound of hexavalent chromium, and thereafterapplying a siccative coating to said continuous chemical coating.

12. The method of coating a zinc surface which treating said surfacewith an aqueous solution of an etching acid for zinc, an oxidizing agentfor zinc, and a soluble compound of cobalt, the acid and oxidizing agentto form an oxide of the zinc of said surface as it is etched, and toproduce a thin continuous visible adha'ent predominantly oxide coatingon said surface in not more than 5 minutes, said coating oxides of zincand cobalt, continuing comprises treating said surface with an aqueoussolution of an etching acid for zinc, an oxidizing agent for zinc, and asoluble compound of nickel, proportioning the acid and oxidizing agentto form an oxide of the zinc of said surface as it is etched and toproduce a thin continuous visible adherent predominantly oxide coatingon said surface in not more than five minutes, said coatin containingoxides of zinc and nickel, continuing the treatment until saidcontinuous coating is formed, thereafter treating the continuouschemically formed coating with a dilute solution of a compound ofhexavalent chromium, and thereafter applying a siccative coating to saidcontinuous chemical coating.

JOHNS. THOMPSON.

